Wireless communication systems are widely used to provide voice and data services for multiple users using a variety of access terminals such as cellular telephones, laptop computers and various multimedia devices. Such communications systems can encompass local area networks, such as IEEE 801.11 networks, cellular telephone and/or mobile broadband networks. The communication system can use a one or more multiple access techniques, such as Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA), Orthogonal Frequency Division Multiple Access (OFDMA), Single Carrier Frequency Division Multiple Access (SC-FDMA) and others. Mobile broadband networks can conform to a number of system types or partnerships such as, General Packet Radio Service (GPRS), 3rd-Generation standards (3G), Worldwide Interoperability for Microwave Access (WiMAX), Universal Mobile Telecommunications System (UMTS), the 3rd Generation Partnership Project (3GPP), Evolution-Data Optimized EV-DO, or Long Term Evolution (LTE).
In coordinated multi-point (CoMP) transmission, transmissions from multiple enhanced Node Bs (eNBs) are made simultaneously to a single User Equipment (UE) or to multiple UEs. Coordination of the transmissions made by the eNBs enable the UE to combine the transmissions to improve high data rate coverage and to increase system throughput in advanced wireless communications systems, such as Long Term Evolution-Advanced (LTE-A). eNBs are also commonly referred to as base stations, base transceiver stations, controllers, access points, and so forth, while UEs are commonly referred to as subscribers, subscriber stations, terminals, mobile stations, for example.
There are generally two CoMP approaches: joint processing from multiple cells (eNBs) and coordinated scheduling/beamforming (CS/CB). In joint processing, there is an assumption that data is available at each transmission point (eNB) in a CoMP cooperating set representing eNBs participating in the CoMP transmission. With joint processing, data is transmitted from more than one eNB at a time. Dynamic cell eNB selection occurs, on the other hand, when the data is transmitted from only one eNB at a time. In CS/CB, the data is available at a serving eNB, and transmission scheduling is coordinated among eNBs within the CoMP cooperating set.
To achieve better channel utilization and increase overall system performance, channel state/statistics/information about a downlink (DL) channel(s) between an eNB and a UE are provided by the UE to the eNB. The channel state/statistics/information provided by the UE enables the eNB to adjust its transmitter to more effectively make use of DL channel conditions.
In general, there are two types of channel state/statistics/information feedback schemes for LTE-A: explicit channel state/statistics/information feedback and implicit channel state/statistics/information feedback. In explicit feedback, an eNB determines the CoMP transmission processing matrix based on the whole or major part of the CoMP channel information. Better CoMP performance can, therefore, be obtained at the expense of a high feedback overhead. With explicit feedback, more information is provided to the eNB to give the eNB more flexibility in scheduling CoMP transmissions. If precoded DL reference signals are used, a selected CoMP transmission scheme can be transparent to the UE. However, uplink (UL) feedback overhead may be high when instantaneous channel information feedback is required.
In implicit feedback, an eNB determines the CoMP transmission processing matrix based on a precoding matrix indication (PMI)/rank indication (RI) recommended by UE. For non-coherent multi-point CoMP transmission, only disjoint PMI/RI information (or individual PMI for cells in CoMP cooperation set) is used. For coherent multi-point CoMP transmission, joint PMI/RI feedback containing individual PMI/RI information and additional inter-cell spatial information or a single joint PMI/RI information is used. Usually joint PMI/RI feedback has a higher overhead than disjoint PMI/RI feedback. With implicit feedback, the UE feeds back channel information based on transmit or receive processing, and incurs less feedback overhead. Implicit feedback may come at decreased scheduling flexibility.
LTE-A is capable of supporting advanced forms of multiple input, multiple output (MIMO), such as higher-order single user MIMO (SU-MIMO) or multi-user MIMO (MU-MIMO). For example, precoding with more than four (4) transmit antennas can be utilized in SU-MIMO and MU-MIMO. More accurate tuning of a transmit beam and/or closed-loop (CL) spatial channel to support of a variety of antenna configurations and propagation scenarios is desired to fully exploit the benefit of more advanced forms of MIMO.